Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator rotor may be electrically coupled to a bi-directional power converter that includes a regulated DC link. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology. Standard power converters typically include a bridge circuit, a power filter, and an optional crowbar circuit. The bridge circuit typically includes a plurality of cells, for example, one or more power switching elements and/or one or more diodes.
In some instances, ground faults can occur in a wind turbine when one or more phases of a conductor become shorted to ground during operation of the turbine. Such ground faults can be damaging to electrical components of the turbine and possibly hazardous to personnel who are present near the turbine. While it is prudent to cease operation of the wind turbine when a ground fault is detected, energy may continue to feed into the fault from the power grid. Since the grid breaker normally has a limited lifetime of operational cycles, the breaker is often operated in such a way that it remains closed even when the wind turbine is not producing power.
Using current feedback signals at various locations in the electrical system of the wind turbine, the wind turbine controller can sense that a ground fault is occurring. Thus, the wind turbine may be shut down and components within the turbine, such as the power converter, may be caused to trip. Such a trip may also open contactors internal to the turbine electrical system. In many cases, opening the internal contactors may prevent the source of electrical energy from the location of the ground fault. However, such operation may not always be the sufficient as the fault current may continually be fed from the power grid if the grid breaker remains closed. Such is especially likely if the amount of ground current is below the level that the grid breaker may use in its hardware to detect a ground fault.
Accordingly, an improved system and method for isolating ground faults in a wind turbine would be advantageous.